Non-ASF product distributions in Fischer-Tropsch synthesis can occur with a gradient in process conditions at the particle or reactor scale, leading to a gradient in chain growth probability a. Weighted summation of local product distributions gives the proper, non-ASF product distribution.The Fischer-Tropsch (FT) process, in which syngas is converted to hydrocarbons, is a key conversion step in one of the most important routes to alternative fuels and chemicals. Various models exist for the selectivity of this reaction. The simplest and most widely used is the Anderson-Schulz-Flory (ASF) model, 1 where the FT reaction is modeled as an addition polymerization reaction with chain growth probability a. The resulting product distribution, depicted as a plot of the logarithmic molar fraction versus the carbon number ('ASF-plot'), is a straight line.However, deviations from this ASF product distribution are regularly observed. Commonly reported deviations from the ASF model are the C1 (methane) and C2 (ethane/ethene) values. Typically, C1 is formed in a greater quantity than that predicted by the ASF distribution, while a much lower amount of C2 is usually measured. 2 Another deviation from the ASF product distribution shape that is encountered in practice is a chainlength-dependent chain growth probability. Instead of a linear trend in an 'ASF-plot' (cf. Fig. 1c), the product distribution shows a double chain growth probability or a curved distribution. 3,4 The fractions of heavier hydrocarbons are in some cases lower than expected based on the ASF model 5 but in most cases they are higher. 2,4,[6][7][8][9][10][11][12][13] The cause of these positive deviations from ASF product distributions is a disputed topic. It has been attributed to flaws in experimentation, 8-10 the existence of multiple chain growth mechanisms 11 or multiple different active sites on the catalyst. 4,12 However, more recent work assumes that olefin readsorption and incorporation is responsible. 2,6,7,13 In this communication we propose an alternative explanation for non-ASF product distributions which is nevertheless based on a single-chain-growth-probability model. Our explanation is based on the process condition dependence of a, where we assume a variable-a model that takes into account the local reactor temperature and the syngas ratio. 14 It has previously been demonstrated that gradients in the process conditions may lead to selectivity gradients. 3,8,14 Temperature and concentration gradients are the most likely variables that cause variations in selectivity. These gradients can take place on the particle scale due to diffusion limitations inside the catalyst particle, as well as on the reactor scale, e.g. due to heat transport limitations in a packed bed reactor. 8,15 We will show that these gradients in process variables can result in an apparent non-ASF Fischer-Tropsch product distribution.Gradients in FT process conditions and resulting chain growth probability and product distributions were modeled on the particle scale and the reac...

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